I know you have been writing for months about your concerns regarding the ability of Nikon to source good sensors (not sure what exactly worries you since you apparently don't own any Nikon glass), but facts at this point seem to indicate they are doing pretty well.
Owning today both a 100mp medium format beast and a 36mp D810, I am really not sure that a 70mp Sony would get me excited to the point of buying one... 
- For "casual shooting", the 45mp of the D850, being close to 4x5 quality, will enable exhibition grade A1 prints,
- For "serious shooting", I am frankly not sure that the different between 45mp and 70mp will be that super visible in prints. Let's not forget that we are now much closer to the peak resolution of many lenses than we were when Nikon went up from 24mp to 36mp. A move from 45mp to 70mp with the same sensor size is going to be incremental for a majority of applications.
It seems likely that the D850 at 45mp with their brilliant 70-200 f2.8 is probably going to be nearly as good as a 70mp Sony with their average 70-200 f2.8.
I may be getting older, but I am finding it more and more difficult to get excited about sensor resolution in 35mm photography. Similarly, a move from 100mp to 150mp in medium format won't prevent me from sleeping at night. We are deep in nitpicking territory and more than ever before, spherical stitching is the only way to get really significant increases of image quality. And for stitching, 45mp and 70mp are very close to identical.
- I would dare to say that if you use T/S lenses to stitch, the 45mp and 70mp will be 99% identical because both will outdo the actual resolution of T/S lenses when shifted a lot,
- For spherical stitching, you are talking about adding a few frames in a pano, with next to zero practical negative impacts.
This is essential about bragging rights in terms of who owns the highest resolution body... and I don't think that Sony will be the only brand with such sensors. At the price point we can expect the a9r to sell for, they will be small series items and it would be in the best interest of Sony to have these sensors sold to Nikon lens owners also.
Cheers,
Bernard
I own a 14-24, which has been used on several different 5D2s, the A7r and the A7r2. Bought it because the Canon 16-35 II was crap. It works with an adapter, but, obviously, is not an electronic aperture lens.
Obviously, a 70MP sensor will be much more demanding on lenses than a 46MP sensor. You may not be able to get the most out of the sensor with all lenses, due to the sensor outresolving the lens. But that's actually the ideal situation, particularly in the absence of an AA filter - it reduces the risk of aliasing artifacts. Furthermore, even though you may not gain much spatial resolution due to lens limitations, you will gain colour resolution, while both chroma and luminance noise will be finer-grained (even if the total noise is the same) and less obtrusive in large prints. Finally, many lenses can take advantage of the resolution, at least in the centre - anything that can use a 28MP APS-C sensor can use a 70MP full-frame sensor.
Yes, the Sony 70-200 f/2.8 is crap. On the other hand, a Sony 70MP camera with a 100-400 GM will likely be far better than a Nikon 46MP camera with an 80-400 on it. And both cameras can take a manual-focus Otus.
With regards to tilt-shift lenses, I'd be very interested to see the new TS-E 50mm. It'd basically be the same as a 50mm lens for medium format in terms of image circle, and those are typically sharp right to the edge.
I just hate having my panos ruined by some random vibration or movement in a single frame, which isn't evident until it's loaded in the RAW converter on a large screen...
There was an interview with Sony a few months back (or was it last year?) where they said they would not be making their best sensors available to other manufacturers. Which would certainly be consistent with no-one else using the 42MP sensor (the Pentax was released after it became available, but still used the old 36MP sensor; also, the 42MP sensor doesn't show up on Sony's own list of commercially-available sensors, whereas the 36MP does). Not sure if this only applies to sensors designed and commissioned by Sony itself, though - it certainly suggests that, if Sony came up with a top sensor for its own cameras, they wouldn't share it, but leaves it open as to whether they'd design one for someone else on commission. Certainly, if Nikon or anyone else came up with their own design and asked Sony to make it, I'd imagine they would - if they didn't, someone else would do it anyway.
At this stage, we don't know if the decision to go with 46MP/9fps was made explicitly to make it an action camera with good cropping potential, or because they couldn't get a higher-resolution sensor (i.e. 'we can't match Canon/Sony's next generation for landscape/studio, but we have the AF system and bandwidth to make it shoot action too, so let's make it the best body out there for that'). It looks like a great camera for wildlife and distant action, but does leave a (relative) hole in Nikon's camera lineup - if anything, it's more in the mould of a D750 successor or super-D750 than a D810 successor. Certainly, many of Nikon's lenses are capable of making use of many more than 46MP. I guess this will be answered by whether they bring out a slow-shooting, ultra-high-resolution body at some stage, although the maximum price of such a body is constrained by the decreasing cost of MF bodies (currently using the Sony 50MP sensor, but there would almost certainly be a higher-resolution sensor in the works for the next generation).
